Speedometer fluctuates when in park and engine off

I sit in my car in the parking lot at school a lot, eating and stuff before my classes. The first time it happened the car was in park but idling, the speedometer was fluctuating between 0-30 mph. I turned the car off and decided it was time to go work on stuff anyway. The 2nd time it did the same thing, the engine was off but the car was on (radio, AC, phone charger, etc.) No other gauges move just the speedometer.

Brief history about my car. A year ago I hydroplaned, jumped a curb, crack the oil pan but wasn't in a place that I could pull over so I killed my engine. I also bent the front passenger side strut and apparently did something to the transmission (it was a tall curb and I was going way too fast) a close friend of a friend that is a mechanic replaced all 3 with junkyard parts. So I know my engine and transmission are a ticking time bomb but have been working fine so far. Except more recently the transmission sometimes seems to have issues shifting while driving and sometimes shifting from PR&D it goes into gear hard (if that makes sense) it's not all the time just sometimes.

In my quest to find out why the speedometer is acting possessed, I found something about a bad speed sensor can cause the transmission to not smoothly change gears. But I have not been able to find any information about the speedometer moving when the engine is off. Every thing that I find is speedometer fluctuating while driving, involving the engine dying while driving, or in park but idling.